all stupidity aside, great barbarians actually sounds like a fun and strategically plausible idea!
imagine your trying to kill barbarians but all of a sudden they basically have a great general boosting them. this definitely would be historical as certain "barbarian" invasions, specially the succesful ones, were so devastating due to their great leadership. attila, the kings of the normans (forget his name?), genghis khan while mongolia is not in the game, timur and a whole lot of historical figures you guys probably know a lot more about than me. strategically speaking, great barbarians would just be great generals and perhaps to make it more balanced, they would only appear as part of a request/mission for city states.
A city state asks for your help in repelling an invasion, an invasion spearheaded by a great barbarian. you get big monetary rewards for destroying the invasion and an even bigger diplomatic reward for defeating the great barbarian. Of course, if you fail to attack the great barbarian may be able to overrun the afflicted city state, in which case he would gain more strength (or maybe none at all) and focus his attention on attacking other civs (perhaps specifically the civ that city-state asked for help) picking up other barbarians from other encampments along the way.
now that i think about it, it may not be thaaaattt strategically balanced, but it would be a fun thing to see be available with an option like "raging barbs"
city states asking for help agaisnt barbarian invasions is quite common. Although the Byzantine empire isn't exactly a city state, the seljuk turks were really destroying them which prompted the first crusade. the greek city states in anatolia asked for help agaisnt the persian armies (okay persians and seljuks are not barbarians but the concept is more or less the same)
someone with super historical knowledge help me out here.